The present invention relates to a method and apparatus for fastening sheet material by self-piercing riveting or clinching. The term xe2x80x9cclinchingxe2x80x9d is also known as xe2x80x9cpress joiningxe2x80x9d or xe2x80x9cintegral fasteningxe2x80x9d.
Methods and apparatus for riveting of the kind in which a self-piercing rivet is inserted into sheet material without full penetration, such that the deformed end of the rivet remains encapsulated by an upset annulus of the sheet material are known.
FIG. 1 is a diagrammatic section of an example of a riveted joint made by such a riveting method in accordance with the invention. A rivet 1 has a head 2 and a shank 3 terminating in an annular edge 4. The shank 3 is initially cylindrical but is flared outwardly into the illustrated shape as the rivet is driven into two overlapping sheets 5,6 located on a suitably shaped die. As shown, the shank and the edge of the rivet 1 remain embedded in the sheet material 5,6 after the rivet has been set.
An improved self-piercing riveting method is described in our European Patent No. 0675774. In this method the sheet material is clamped in the region around the rivet insertion with substantial force prior to the commencement of insertion of the rivet and then during rivet insertion. Clamping is applied between a nose of the riveting machine and the die in the region around the rivet insertion location so that there is minimal distortion of the sheet material during the riveting operation. The process is achieved by using two concentric and independently operable hydraulic cylinders. An outer cylinder applies the clamping force and the inner cylinder applies force to insert the rivet. This method has been proved to increase the strength of the riveted joint and reduce the depth of the annular valley 7. However, the relatively high level of clamping force required to achieve the improved joint characteristics means that a significant pressure of hydraulic fluid or a heavy-duty spring is required to apply the force. Furthermore, if reaction forces within the joint resulting from rivet insertion exceed the clamping force, the nose will be pushed back up away from the die. This results in a reduction of the potential residual compressive stress that could be imparted to the region around the rivet.
Although the hydraulic riveting process described in our aforementioned patent is effective in producing distortion free joints that have improved fatigue life and reduced standard deviation in static strength, it is does require a relatively bulky rivet setter. Moreover, the two-stage process of applying the clamping force and then applying the rivet insertion force adds to the cycle time.
In other riveting apparatus the two-stage process is replaced by a single stage operation in which the clamping force is provided by the compression of a single internal spring between the nose and the actuator of the rivet setter (hereinafter referred to as xe2x80x9cspring clampingxe2x80x9d). In a single smooth stroke the clamping force is applied to the sheet material before insertion of the rivet and is increased as the actuator descends and compresses the spring. After predetermined travel distance the punch fitted to the end of the actuator comes into contact with the rivet and insert it into the she material. Continued compression of the spring occurs during insertion of the rivet so that the clamping force continues to increase.
Spring clamping of this kind has disadvantages in several respects. First, tests have established that the fatigue life of a riveted joint produced according to the method is significantly reduced in comparison that of a joint produced using the two-stage process. Secondly, the life of the spring is relatively short unless it is of a considerable size (and therefore very bulky in comparison to a hydraulic clamp). The life of a spring is dependent on its initial load, its final (fully compressed) load and the length of travel between these two positions. Since effective clamping of sheet material for self-piercing riveting requires forces of around 4 to 8 kN and rivets can be in excess of 15 mm in length the spring must be designed to withstand the repeated application of such loads over such stroke lengths. The life of such a spring is typically 100,000 cycles or less. Such rapid degradation of the spring results in the production of joints of variable and unpredictable quality. The riveting apparatus and process thus require stringent monitoring systems and frequent preventative maintenance. An alternative option is to use a larger spring with a better specification but this is usually too bulky to accommodate in a rivet setting apparatus of reasonable size. Thus the life of the spring is usually compromised.
In tests conducted by the applicant, a pair of aluminum sheets (5000 series) of 2 mm thickness were riveted together using a two-stage hydraulic process hydraulic clamping force applied prior to rivet insertion and another pair of identical sheets were riveted using spring clamping. The respective clamping forces were of identical magnitude. The joint produced with hydraulic clamping was found to have a fatigue life of around 1.2 million cycles when tested at 40% of the tensile load to failure (970 lb in this case). The fatigue test applied a tensile load cycling between 388 lb (40% of the tensile failure load) and 38.8 lb (i.e. 10% of the maximum) at a frequency of 20 Hz. In contrast, the joint produced with spring clamping had a fatigue life of only 0.6 million cycles. A further test established that a riveted joint produced without any significant clamping force (i.e. the force applied is sufficient only to hold the nose of the rivet setter steady against the sheet material during the riveting operation but has no effect on the flow or displacement of material of the sheet during rivet insertion) had a fatigue life of 1.1 million cycles.
Self-piercing riveting is closely related to clinching in which two sheets of metal are deformed into locking engagement using a punch-and-die combination. An improved clinching method is described in our European Patent No. 0614405. In this method a hollow rivet or tubular slug is inserted into a clinched joint between sheets and the inner end of a shank of the rivet is outwardly deformed within the clinched joint in such a way that it does not penetrate the panels.
In both clinching and self-piercing riveting methods a C-frame is used to support the riveting apparatus and die. A lower limb of the C-frame supports the die and, in use, deflects a certain distance during the riveting operation as a result of the rivet insertion and clamping forces. This means that in hydraulic clamping systems top-up hydraulic fluid is generally required to maintain the required level of clamping. The slow response of hydraulic fluid systems to the demand for extra loading leads to relatively long cycle times.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages and to provide for an improved method and apparatus for fastening sheet material by self-piercing riveting or clinching.
According to a first aspect of the present invention there is provided a method for inserting a fastener into sheet material comprising inserting the fastener into at least one sheet without full penetration such that a deformed end of the fastener remains encapsulated by an upset annulus of the sheet material, the sheet material being disposed between a nose and a die of fastening apparatus and the fastener being inserted into the sheet material by means of a plunger that is reciprocal relative to the nose, characterised in that, during a first stage of fastener insertion the sheet material around the fastener insertion location is displaced towards the fastener by virtue of its insertion, and thereafter during a second stage (being after said first stage) of fastener insertion a clamping force of sufficient magnitude is applied between the nose and the die in the region around fastener insertion location so as substantially to prevent flow of displaced sheet material away from the fastener.
The clamping of the sheet at the last stage of insertion of the fastener in this way ensures that favourable compressive stresses are built into the region around the fastener insertion location and, in the case where one or more sheets are being joined, also ensures that fatigue performance of the joints significantly improves in comparison to joints produced by conventional fastening methods. The application of the clamping force is timed to occur just as a head of the rivet being inserted is pressed flush and the material that was previously flowing into the die changes direction and begins to be pushed out of the die. Restraining this reverse flow is the key to creating favourable compressive stresses in the material around the rivet head and shank. The clamping force is sufficient to flatten any distortion that occurred during the initial stages of no or low clamping. It also brings the sheet materials into intimate contact with each other so as to provide a compact gap-free joint.
The absence of any significant clamping force during the initial stages of fastener insertion allows significant distortion to occur in the sheet material in the region surrounding the insertion location. The level of distortion is dependent on the number of sheets being joined and their thicknesses. Tests conducted by the applicant established that this distortion can be flattened by applying a significant clamping force to the material surrounding the inserted fastener just as it is pressed flush with the sheet material. Moreover, it was established that the fatigue life of the joint was surprisingly enhanced.
The clamping force is preferably provided by compression of a compressible member that is disposed between the plunger and the nose. The compressible member may be a plurality of disc springs or other resilient element.
Preferably the first stage of the fastener insertion comprises entry of the fastener into the sheet material and the second stage comprises the fastener being pressed substantially flush with an upper surface of the sheet material. The clamping force applied during the second stage of fastener insertion ideally increases until the fastener is fully inserted. During the first stage of fastener insertion preferably no clamping force is applied to the sheet material in the region around the fastener insertion location.
During the first stage of fastener insertion the nose of the apparatus may be biased into abutment with the sheet material so as to provide stable contact between the two.
The second stage of fastener insertion is preferably when a head part of the fastener is pressed flush with an upper surface of the sheet material.
According to a second aspect of the present invention there is provided apparatus for inserting a fastener into sheet material without full penetration such that a deformed end of the fastener remains encapsulated by an upset annulus of the sheet material, said apparatus comprising a nose in which is disposed a reciprocal plunger, means for feeding fasteners successively to the nose for insertion by the plunger into the sheet material, a die aligned with the plunger for deforming the fastener being inserted, the sheet material being disposed between the nose and die during the fastening operation, characterised in that there is provided means for applying a clamping force at a predetermined point during insertion of the fastener, said means for applying the clamping force allowing the sheet material around the fastener insertion location to be displaced by the fastener towards the fastener during a first stage of fastener insertion, and thereafter during a second stage (being after said first stage) of fastener insertion applies a clamping force of sufficient magnitude between the nose and the die in the region around fastener insertion location so as substantially to prevent flow of displaced sheet material away from the fastener.
According to a third aspect of the present invention there is provided a panel clinching method wherein two or more sheets of material are deformed into locking engagement, the sheet material being disposed between a nose and a die of fastening apparatus, the sheet material being deformed by means of a plunger that is reciprocal relative to the nose, characterised in that, during a first stage of deformation, the sheet material around the deformation region is displaced towards the plunger by virtue of its insertion into the material, and thereafter during a second stage (being after said first stage) of deformation a clamping force of sufficient magnitude is applied between the nose and the die in the region around the deformation so as substantially to prevent flow of displaced sheet material away from the plunger.